A conventional switched power supply employs a full wave rectifier for rectifying an AC input supplied by a power line, and a power regulator circuit coupled with the rectifier for delivering a DC voltage of substantially constant magnitude to a load.
The power supply provides the rectified AC line input to the load during only the portion of each of its cycles when its voltage is above the desired DC output voltage. The power supply also charges a storage capacitor during that high-voltage portion of each cycle, and discharges the capacitor for energizing the load during the remaining, low-voltage portion of each cycle. For this reason, the power regulator has a solid state switch, e.g., a field effect transistor ("FET"), responsive to a switching control signal from a control circuit for coupling to the load in alternation the rectified AC line voltage and the discharged voltage from the capacitor.
Conventionally, the control circuit monitors the AC line using a line sense transformer, and, in response, controls the duty cycle of the switching control signal. The switching control signal typically changes between high and low voltage levels in a digital manner so as to respectively turn on and off the FET at the desired times during each AC line voltage cycle.
Some switched power supplies are adapted for operating with high, i.e., near unity (about 0.95-1.0), power factors. Commonly, such a high-power-factor ("HPF") power supply modulates the duty cycle of the switching control signal to achieve a high power factor by forcing the current drawn by the power supply to be in phase with the supplied line voltage. Such an approach is disclosed in U.S. Pat. No. 4,677,366, which issued Jun. 30, 1987 to Wilkinson, and in an article entitled "High Power Factor Pre-Regulators For Off-line Power Supplies" by Lloyd H. Dixon, Jr., which appeared in Unitrode Switching Regulated Power Supply Design Seminar Manual, 1990, by Unitrode Corporation, pages I2-1 through I2-16.
In the control circuit of the conventional HPF power supply, a voltage error amplifier produces an output that is a function of the difference between the output of the power supply and a reference voltage, and a switching analog multiplier produces an output that is proportional to the output of the voltage error amplifier multiplied by the instantaneous line voltage and divided by the square of the root-mean-square ("RMS") line voltage. Thus, the conventional control circuit has a square law dependency on the line voltage. The control circuit also has a current regulator for producing a current error signal responsive both to the current flowing in the power supply and to the output of the multiplier. A pulse-width modulator modulates the duty cycle of the control signal in response to the current error signal. The resulting control signal is provided to the control terminal of the semiconductor switch, e.g., the gate of the FET.
While HPF power supplies of the type just described are generally suitable for their intended purposes, they generally have limited input dynamic ranges. This is due to the fact that, in controlling the power factor, such power supplies generally use the square of the RMS line voltage divided by resistance. Unfortunately, operational amplifiers ("OP AMP's") typically employed in the control circuits are limited in their output to a 10-volt span. Consequently, the "voltage squared" term used by such control circuits requires that the OP AMP inputs representing the RMS line voltages be limited to the square root of ten, or about 3.1 volts. If the nominal input is one and a half volts, the control circuit can only exhibit a 2:1 input dynamic range. Thus, the conventional HPF power supplies can not accommodate RMS line voltages that extend beyond the range of, e.g., 150 volts to 300 volts.
To make matters worse, non-linearities near the limits of the input dynamic range further restrict the RMS voltages over which the prior art HPF power supplies can operate effectively.
Another drawback to the prior art HPF power supplies relates to the provision of the reference voltages used in the control circuits. Conventionally, the reference must be set to represent the desired DC level of the output from the power supply. The DC output level from the power supply can vary depending on the RMS amplitude of the AC line voltage. The RMS amplitude can vary as a result of line conditions, and can vary from country to country as a result of differing power distribution standards.
All of these factors must be taken into account in the prior art HPF power supplies in establishing the reference voltages. Thus, the prior art HPF power supplies typically utilize costly and complicated auxiliary voltage supplies to generate the reference voltages. Moreover, the prior art HPF power supplies typically have to use different line sense transformers in their control circuits for the different line power levels used in different parts of the world.
Furthermore, with any significant change in the AC line voltage, the prior art HPF power supplies require that the reference voltages be re-calibrated. It would be desirable to avoid the necessity to provide an auxiliary power supply and to eliminate or at least greatly reduce the need for transformer change-overs and re-calibration of the references.
Yet a further drawback of prior art switched power supplies relates to the harmonics they produce. Since the power supplies operate at higher frequencies than the power lines (e.g., about 100 kilo-Hertz compared with 60 Hz on the power lines), harmonics fed back onto the power lines can present special problems. It would be desirable to control the power factor with greater precision than in prior art arrangements so that the current drawn by the power supply more nearly tracks the waveform of the AC voltage so as to minimize harmonics.